The present invention relates to speech decoders for synthesizing speech by using indexes received from the encoding side and, more particularly, to a speech decoder which has a postfilter for improving a speech quality through control of quantization noise superimposed on synthesized signal.
As a system for encoding and transmitting a speech signal satisfactorily to certain extent at low bit rates, a CELP (Code-Excited Linear Prediction) system is well known in the art. For the details of this system, it is possible to refer to, for instance, M. Schroeder and B. Atal "Code-excited linear prediction: High quality speech at very low bit rates", Proc. ICASSP, pp. 937-940, 1985 (referred to here as Literature 1) and also to W. Kleijin et al "Improved speech quality and efficient vector quantization in SELP", Proc. ICASSP, pp. 155-158, 1988 (referred to here as Literature 2).
FIG. 1 shows a block diagram in the decoding side of the CELP method. Referring to FIG. 1, a de-multiplexer 100 receives an index concerning spectrum parameter, an index concerning amplitude, an index concerning pitch and an index concerning excitation signal from the transmitting side and separates these indexes. An adaptive codebook unit 110 receives the index concerning pitch and calculates an adaptive codevector z(n) based on formula (1). EQU z(n)=.beta.(n-d) (1)
Here, d is calculated from the index concerning pitch, and .beta. is calculated from the index concerning amplitude. An excitation codebook unit 120 reads out corresponding codevector S.sub.j (n) from a codebook 125 by using the index concerning excitation, and derives and outputs excitation codevector based on formula (2). EQU r(n)=.gamma..multidot.s.sub.j (n) (2)
Here, .gamma. is a gain concerning excitation signal, as derived from the index concerning amplitude. An adder 130 then adds together z(n) in formula (1) and r(n) in formula (2), and derives a drive signal v(n) based on formula (3). EQU v(n)=z(n)+r(n) (3)
A synthesis filter unit 140 forms a synthesis filter by using the index concerning spectrum parameter, and uses the drive signal for driving to derive a synthesized signal x(n) based on formula (4). ##EQU1## Here, .alpha.'.sub.i (i=1, . . . , M, M being the degree) is a linear prediction coefficient which has been restored from the spectrum parameter index in a spectrum parameter restoration unit 145. A postfilter 150 has a role of improving the speech quality through the control of the quantization complex noise that is superimposed on the synthesized signal x(n). A typical transfer function H(z) of the postfilter is expressed by formula (5). ##EQU2## Here, .gamma..sub.1 and .gamma..sub.2 are constants for controlling the degree of control of the quantization noise in the postfilter, and are selected to be 0&lt;.gamma..sub.1 &lt;.gamma..sub.2 &lt;1.
Further, .eta. is a coefficient for emphasizing the high frequency band, and is selected to be 0&lt;.eta.&lt;1. For the details of the postfilter, it is possible to refer to J. Chen et al "Real-time vector APC speech coding at 4,800 bps with adaptive postfiltering", Proc. IEEE ICASSP, pp. 2,185-2,188, 1987 (referred to here as Literature 3).
A gain controller 160 is provided for normalizing the gain of the postfilter. To this end, it derives a gain control volume G based on formula (6) by using short time power P.sub.1 of postfilter input signal x(n) and short time power P.sub.2 of postfilter output signal x'(n). EQU G==.sqroot.(P.sub.1 /P.sub.2) (6)
Further, it derives and supplies gain-controlled output signal y(n) based on formula (7). EQU y(n)=g(n).multidot.x'(n) (7)
Here, EQU g(n)=(1-.delta.)g(n-1)+.delta..multidot.G (8)
Here, .delta. is a time constant which is selected to be a positive minute quantity.
In the above prior art system, however, particularly in the postfilter the quantization noise control is dependent on the way of selecting .gamma. 1 and .gamma. 2 and has no consideration for the auditory characteristics. Therefore, by reducing the bit rate the quantization noise control becomes difficult, thus greatly deteriorating the speech quality.